Given a number n, determine what the nth prime is.
By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.
Go to the root of your PHP exercise directory, which is <EXERCISM_WORKSPACE>/php
.
To find the Exercism workspace run
% exercism debug | grep Workspace
Get PHPUnit if you don't have it already.
% wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
% chmod +x phpunit.phar
Execute the tests:
% ./phpunit.phar nth-prime/nth-prime_test.php
A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
<?php
require 'nth-prime.php';
class NthPrimeTest extends PHPUnit\Framework\TestCase
{
public function testFirstPrime()
{
$this->assertEquals(2, prime(1));
}
public function testSecondPrime()
{
$this->markTestSkipped();
$this->assertEquals(3, prime(2));
}
public function testSixthPrime()
{
$this->markTestSkipped();
$this->assertEquals(13, prime(6));
}
public function testBigPrime()
{
$this->markTestSkipped();
$this->assertEquals(104743, prime(10001));
}
public function testZeroPrime()
{
$this->markTestSkipped();
$this->assertEquals(false, prime(0));
}
}
<?php
function prime(int $n): int {
if ($n <= 0) {
return false;
}
for ($i = 2, $count = 0; $count < $n; $i++) {
if (gmp_prob_prime($i)) {
$count++;
}
}
return $i-1;
}
A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.
Here are some questions to help you reflect on this solution and learn the most from it.
Community comments
Okay I know I'm cheating with gmp ;-) This elaborated answer is great read.